1. Field of the Invention
The present invention relates to electrostatic discharge devices of the surface mount type and a method of fabricating such devices and, more particularly, to an electrostatic discharge device of the surface mount type designed to be easily installed on a printed circuit board through a surface mounting process and used for protecting electronic circuits or electronic parts from electrostatic damage, and to a method of easily and simply fabricating such devices through a ceramic laminating process.
2. Description of the Prior Art
As well known to those skilled in the art, electronic circuits or electronic elements (herein below commonly referred to simply as xe2x80x9celectronic circuitsxe2x80x9d) of a variety of electronic apparatuses, such as sensors, are electrostatically impacted by an application of static electricity, such as an instantaneously applied high voltage, during an operation. The electronic circuits may be thus seriously damaged, causing operational errors, losing their operational functions, and being broken. As the electronic circuits have become complicated in their construction in accordance with the rapid development of electronic apparatuses in recent years, the electronic circuits become more sensitive to surges. Due to such sensitivity of the electronic circuits to surges, the electronic circuits may be more easily and frequently damaged by static electricity during an operation.
In an effort to overcome such electrostatic damage to the electronic circuits, several techniques have been actively studied and developed in recent years. As an example of such techniques, ESD devices (electrostatic discharge devices) have been proposed and used widely.
An example of conventional ESD devices is shown in FIGS. 1a and 1b. FIG. 1a shows an arrangement of the conventional ESD device connected to both an antenna and an electronic circuit. FIG. 1b shows a cross-section of the ESD device.
As shown in FIG. 1a, when the electronic circuit of an apparatus receives signals from an antenna through a signal transmission wire, the circuit may be impacted by an instantaneous application of a high voltage signal. In order to prevent such an application of high voltage signal to the circuit, an ESD device 30 is installed on the signal transmission wire in parallel to the circuit so as to protect the circuit from such a high voltage signal by performing a plasma discharge of static electricity.
As shown in FIG. 1b, the conventional ESD device 30 comprises a hollow cylindrical case 31, with two holed disc covers 32a and 32b set in opposite ends of the case 31 to close the ends to form a cavity within the case 31. Plasma discharge gas is fed into the case 31 through the holes of the two covers 32a and 32b to fill the cavity of the case 31. Two signal transmission wires are inserted into the opposite ends of the case 31 through the holes of the two covers 32a and 32b to reach predetermined positions within the cavity, prior to sealing the gaps between the holes and the wires using insulators 33a and 33b. 
When static electricity, having a potential higher than the ionization potential of the plasma discharge gas contained in the ESD device 30, is introduced into the ESD device, the plasma discharge gas is ionized to perform plasma discharge, thus reducing the voltage of the signal transmitting wires. This protects the electronic circuit from high voltage static electricity surges.
However, the conventional ESD device 30 is manufactured through a complex process. That is, the process of producing the ESD device 30 comprises the steps of setting the two holed covers in the opposite ends of the hollow cylindrical case, feeding plasma discharge gas into the case through the holes of the two covers, inserting two signal transmitting wires into the case through the holes of the two covers, and sealing the gaps between the holes and the wires using insulators. Such a complex manufacturing process undesirably increases the manufacturing cost of the ESD devices. Another problem experienced in the conventional ESD device resides in that the ESD device is too large in its dimension, thus undesirably and excessively consuming the surface area of a printed circuit board (PCB).
FIGS. 2a, 2b and 2c are a perspective view, a plan view, and a sectional view of a conventional ESD device in accordance with another embodiment of the prior art. As shown in the drawings, this conventional ESD device is designed to be improved in its welding-sealed structure including a cylindrical discharge tube 10 containing ionization gas therein. In the ESD device, the discharge tube 10 comprises a cylindrical case 11, which is made of a conductive metal and is provided with a plurality of axial holes 12 extending in parallel to the axis 15 of the case 11. Two insulating tubes 16 are set within each of the axial holes 12 such that the two tubes 16 are inserted into each hole 12 from the upper and lower ends of the hole 12 to form a cavity 20 between the inside ends of the two tubes 16. Ionization gas fills the cavity 20 before an electrode 19 penetrates the communicating holes of the two tubes 16 while passing through the cavity 20. The above-mentioned construction of this conventional ESD device is expressed in U.S. Pat. No. 5,726,854 in detail.
When a high voltage is applied to the ESD device 10 during an operation, the ionization gas within the case 11 is ionized and responds to the high voltage surge acting on the junctions of the electrodes 19 and the grounds, thus forming conductive passages at the gaps between the electrodes 19 and the case 11 and bypassing the high voltage to the grounds. Therefore, the ESD device 10 protects circuit elements and semiconductor chips operated in conjunction with status reaction sensors from such a high voltage surge.
The above-mentioned ESD device 10 is advantageous in that it is possible to selectively use the electrodes during an operation. However, this ESD device further complicates the process of manufacturing the ESD devices and increases the production costs of the devices. Another problem of this ESD device resides in that it is too large in its dimension, thus undesirably and excessively consuming the surface area of a PCB.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an electrostatic discharge device (ESD device) of the surface mount type, which is designed to be easily installed on a PCB through a surface mounting process and is used for protecting electronic circuits or electronic parts from electrostatic damage, and also provides a method of easily and simply fabricating such ESD devices through a ceramic laminating process.
In order to accomplish the above object, the present invention provides an electrostatic discharge device of the surface mount type, comprising: an upper cover plate made of an insulating material; a middle insulating plate made of an insulating material and laminated on the lower surface of the upper cover plate, and having a discharge opening, with first and second discharge terminals formed in the middle insulating plate at opposite edges of the discharge opening; and a lower cover plate made of an insulating material and laminated on the lower surface of the middle insulating plate, and hermetically sealing the discharge opening of the middle insulating plate in cooperation with the upper cover plate, the lower cover plate having a second signal electrode brought into electric contact with the first discharge terminal of the middle insulating plate, and a second ground electrode brought into electric contact with the second discharge terminal of the middle insulating plate, whereby discharge gas fills the discharge opening of the middle insulating plate sealed by the upper and lower cover plates.
In the electrostatic discharge device, the middle insulating plate further comprises a first signal electrode bringing the first discharge terminal of the middle insulating plate into electric contact with the second signal electrode of the lower cover plate; and a first ground electrode bringing the second discharge terminal of the middle insulating plate into electric contact with the second ground electrode of the lower cover plate.
This electrostatic discharge device is easily installed on a PCB through a surface mounting process, and is easily and simply produced through a ceramic laminating process at a low production cost.